Technique Measures Muscle Contraction At Molecular Level

CHAMPAIGN, Ill. - A measurement technique originally designed for studying interactions within molecules of DNA has been used to examine muscle movement at the molecular level, says a University of Illinois researcher who developed the procedure.

Called luminescence resonance energy transfer, the technique is shedding light on how the two major components of muscle -- proteins of myosin and actin ­ interact to create movement.

"Myosin is a little molecular motor that converts chemical energy into mechanical motion," said Paul Selvin, a professor of physics at the U. of I. "To do this, there are tiny myosin heads that stick out and grab the actin. Muscle contraction occurs when the myosin heads rotate and pull on the actin filaments."

The details of how the myosin heads move are not well understood, however. "The leading model says the myosin heads act like little lever arms that pull on the actin in much the same way that an oar pulls on water," Selvin said. "Our measurements provide direct evidence in support of this lever-arm model."

Selvin and his colleagues -- professor Roger Cooke at the University of California at San Francisco, professor Ralph Yount and postdoctoral research associate Handong Li at Washington State University, and postdoctoral research associate Ming Xiao and graduate student Gregory Snyder at the U. of I. -- recently measured the molecular movement of probes precisely positioned on the myosin head.

"At the molecular level, the shape changes are so small -- 2 to 10 nanometers, or 10,000 times smaller than the width of a human hair -- you can't see them by eye or even with a regular microscope," Selvin said. "Instead, we label various parts of the myosin with different dyes and watch how the light emitted from the dyes changes as the protein moves."

Selvin's measurement technique is a new twist on a 40-year-old technique called fluorescence resonance energy transfer. Instead of using conventional dyes, however, Selvin uses a rare-earth metal atom in combination with a regular dye. The metal atom is attached to the myosin molecule and lights up with a distinct color when excited by a laser beam. If a regular dye is attached to a different part of the molecule, some of the metal atom's energy can be transferred to the dye, which then emits light of its own distinct color. By looking at the relative signal strengths of the two probes, the researchers can tell how much energy is transferred, and from that determine the distance between them.

"We know the atomic structure of the myosin head, and we know where we put the probes, so we know what the distance is initially," Selvin said. "After the myosin head rotates we measure another distance and then calculate the corresponding shape change by using simple geometry."

The researchers described their measurement technique and the changes that occur within the myosin head in the Dec. 22 issue of the Proceedings of the National Academy of Sciences.